THE CHROMOSOMES 135 



there is no explanation as to why the factors in a 

 group remain together in sections as explained on 

 page 66. If it is supposed that the chromosomes 

 break only once or twice, and that linkage represents 

 the holding together of the pieces, then one is forced 

 to assume that the breaking up is the same in both 

 members of a pair, yet entirely inconstant in different 

 cells; for otherwise the reunion of the fragments 

 would lead to duplication or loss of whole sections 

 of the chromosomes, and all order would soon be lost. 

 A large amount of data relating to sex linked char- 

 acters has shown that the sex chromosomes must 

 remain intact as often as they break apart, and even 

 when they break apart this takes place, as a rule, at 

 only one place. 



The interpretation of Mendelian inheritance on a 

 chromosomal basis by no means excludes the possi- 

 bility that there may be other forms of inheritance 

 depending on other cell materials. Although the 

 cytoplasm is essential for the development of the 

 organism, and is transmitted by the egg to each new 

 generation, its materials do not perpetuate themselves 

 unchanged as do the chromosomes, and are therefore 

 really not hereditary. There are, however, certain 

 bodies carried by the protoplasm, such as plastids 

 (possibly also chondriosomes) , which like the chro- 

 matin are able to grow and divide, and hence might 

 have the power to peTpetuate themselves unchanged 



